Electric pick-up



May 2, 1933. J. M. KENDALL 1,906,615

ELECTRIC?- IPICK-UP' Filed April 21', 1928.

Inventor- I Janwes M. Kc-snclaH. Joy

Hi8 Attorney.

Patented May 2, 1933 UNITED *sr TEs.

PArE-Nr oFtFIcEfl JAMES M. KENDALL, or scnENEq'rnnY, NEW YORK, ASSIGNOR.'].0 GENERAL ELECTRIC COMPANY, 'A 'COBPORATIONOF. NEW vonx ELECTRICPIcK UP Application filed April 21,

My invention relates to apparatus for producing sound, and has for-itsprincipal object the provision of an improved sound producing apparatuswhich possesses low impedance, has a comparatively wide range offrequency response, and operates with greater flexibility than the soundproducing apparatus heretofore provided. 1

Various types of apparatus for the interconversion of sound andelectrical vimpulses have been provided in the past. Many of theseapparatus :comprise a mag-, netic armature which ismoved either inaccordance with electrical impulses dependent on sound or in accordancewith the contour of a sound record. In the'operationofsuch apparatusdifliculty is frequently encountered because of the: sound distortionwhich results from the tendency of the armature to vibrate at afrequency dependent on its free vibration. period. Other difiicultiesencountered are undue wear of" the sound record and failure of theapparatus to respond f' properly to lowfrequency impulses. In accordancewith my invention, these difficulties are minimized or altogether.avoided by the provision of a sound producing apparatus which includesan armature arranged. to-move with a high degree offlexibility and anoperating coil of much lower impedance thanthe operating. coilsheretofore utilized in such devices.

My invention will be better understood from the following descriptionwhen. considered in connection with the. accompanying drawing anditsscope will be pointed out in the appended claims.

Referring to the drawing, 'Figs. 1 and 2 illustrate aphonographicreproducer or electrical pick-up devicewherein my inventionhas been embodied; and Fig. 3 illustrates a system wherein thisreproducer may be utilized. I

The sound producing appartus or phonographic reproducer illustrated bythe drawing includes a magnet 10 provided with bipartite pole pieceslland 12which are arranged to support and partlyenclose a low impedancecoil 13' interposed between the upper extensions Hand 15 and the lower1928. Serial No. 271,813.

extensions 16 and 17 of the pole piecesg A movable armature I8flattened.at its upper 'end and provided with shaft 19 and with and. 15 of thepole pieces 11 and 12 either in response to movementof a stylus attachedto the lower end of the armature-by means of a thumb screw 21 or inresponse to the electrical impulses supplied to the coil13 throughtheleads 22 and 23. I

In order to center the upper end of the armature 18' between the upperpole piece extensions 14,.a11d15 and to control the movement of thearmature, a resilientblock I T 2 1 of rubber or the like and anadjusting plate 25 are provided. The block 245 may be grooved at itslo'wersurface to receive the upper end of the armature, preferably restson small ledges 40 at the top oftheupper pole piece extensionsll andl5so asto oppose little frictional force to the movement of thearmature, and is clamped to these ex tensions by means of screws 26 andthe adjusting plate 25 which may be moved slightly in one directiouorthe other for the purpose of centering the armature between the polepieces. It will be apparent that'the resilient member 24 is subject tosubstantially no compressive and frictional forces due to thefact thatit is held by its edges which are clamped between the ledges 40 and theadjusting plate 25. With this construction, the armature moves through adistance which is small as compared to the dimensions of the rubberblock along the direction of travel ofthe armature and moves freely. .Asclearly indicated by Fig. 1, the casing ofthe apparatusincludes a member27 to which theam agnet 10-is attached by means of a screw 28fland a'clampingplate '29 .and towhich the pole pieces 11' and 12 are attachedby means of bolts 30 and 31. This oasingalso includes a cover member 32which 'isattached to the clamping-plate 29 by means of screws 33. Itwill be seen that pole pieces 11, 12 areprovided with upper surfaceswhich are in engagement with the surfaces of the bottom ends of magnet10. This construction avoids the necessity of bolts between the polepieces and magnet and permits lateral adjustment of the pole pieces.Assembly of the unit is simplified since means entirely independent ofthe contacting surfaces of the pole pieces and magnet maintain themagnet and pole pieces in proper relation. The magnetic attraction holdsthe magnet to the pole pieces, this being supplemented by holding meansassociated with the casing, such as clamping plate 29.

As will be apparent from Fig. 3, the coil 13 is connected to currentsupply terminals 34 through a volume control potentiometer 35 and atransformer 36 which may possess a high transformation ratio.

It will be readily understood that, with the exception of the lowimpedance coil 13, the electrical features of the improved ap paratusare similar in some respects to some types of sound reproducingapparatus heretofore provided. The utilization of this low impedancecoil is advantageous in that it makes possible the use of a small coiland a short armature which possesses a comparatively low moment ofinertia and has a better response, especially at low frequencies, thanthe armature heretofore utilized.

Other practical advantages resulting from the use of a low impedancecoil will be apparent from the following considerations. With the highimpedance sound producing apparatus heretofore utilized, it has beennecessary to use a volume control potentiometer which had a resistanceof approximately 20,000 ohms and to ground the metal part of theapparatus. In the case of a low impedance coil having about 300 turnsand an impedance of approximately 26 ohms at 1000 cycles, a volumecontrol potentiometer of only 60 ohms is satisfactory and. grounding ofthe metal parts is rendered unnecessary. The comparative cost of thesetwo features for the old and improved apparatus is in the ratio of 12to 1. Further advantages of the low impedance coil are a reduction ofalternating current hum and the simplification of the problem oftransmitting energy from the reproducer to the loud speaker.

The flexible armature mounting made possible by the member 24 has theadvantages of ease in the adjustment of the apparatus, greater amplitudeof the armature motion in the air gap, higher voltage per turn, greaterreliability in operation due to the fact that the centering and dampingmember, because of the extremely small amount of compression, does nottake a permanent set and operates with substantially no friction anduniformity in production due to the fact that the comparatively largedamping member may be cut with greater accuracy than the comparativelysmall resilient stops heretofore used, and less wear on the phonographrecord both at high and low frequencies.

It has been found as the result of extensive tests that the aboveadvantages are realized if the coil 13 has from 50 to 1200 turns and hasan impedance which may vary from 9 to 4640 ohms at 10000 cyclesdepending on the number of turns. The armature 18 may vary somewhat inlength but preferably has a moment of inertia which is less than .15 gmcms. and may readily be made as low as .08 gm cmsF. For convenience ofeXpl'ession the terms low impedance and short are used herein todesignate a coil and an armature having these characteristics.

The embodiment of the invention illustrated and described herein hasbeen selected for the purpose of clearly setting forth the principlesinvolved. It will be apparent however that the invention is susceptibleof being modified to meet the different conditions encountered in itsuse and I therefore aim to cover by the appended claims allmodifications within the true spirit and scope of the invention.

WVhat I claim as new and desire to secure by Letters Patent of theUnited States is:

1. In a sound producer, the combination of a pair of magnetic polepieces, a relatively broad resilient member clamped to said pole piecesalong limited portions of said member adjacent opposing edges thereof,said member having a portion of sub stantial extent intermediate saidclamped portions, said intermediate portion being unconfined over bothupper and lower surfaces thereof, whereby said intermediate portion ofsaid member is subjected to substantially no compressive and frictionalforce, and an armature pivoted between said pole pieces near one of itsends and mechanically coupled to said intermediate portion of saidresilient member at the other of its ends.

2. In a sound producer, the combination of a pair of magnetic polepieces, a resiiient member, an armature pivoted between said pole piecesnear one of its ends and mechanically coupled to said resilient memberat the other of its ends, and a low impedance coil interposed betweensaid pole pieces and arranged to surround said armature.

8. The combination of a pair of magnetic pole pieces, a resilientmember, an armature pivoted between said pole pieces near one of itsends and mechanically coupled to said resilient member at the other ofits ends, a

coil surrounding said armature, and a volume control. potentiometerhaving a resistance of less than one hundred ohms connected to theterminals of said coil.

4. In a sound producer, the combination.

of a pair of magnetic pole pieces, a resilient member, and an armaturepivoted between said pole pieces at one of its ends and mechanicallycoupled to said resilient member at the other of its ends, said armaturehaving a moment of inertia which is less than .15 gm. cms

5. In a sound producer, the combination of a pair of magnetic polepieces,-a resilient member, an armature pivoted between said pole piecesat one of its ends and mechanically coupled to said resilient member atthe other of its ends, and acoil surrounding said armature, said coilhaving an impedance between 9 and 4640 ohms at 10000 cycles. 1

6. In a sound producer, the combination of a pair of magneticpolepieces, an armature pivoted between said pole pieces near one of itsends, and a relatively wide resilient block clamped to said pole piecesalong relatively narrow portions of said member adjacent opposing edgesthereof and bridging said pole pieces, said member having a centralportion unconfined over both upper and lower surfaces thereof, saidcentral portion of said resilient block engaging the other end of saidarmature.

7. In a sound producer, the combination of a pair of magnetic polepieces, a single resilient block clamped along relatively narrowportions thereof adjacent two opposite edges thereof on the tops of saidpole pieces, said block having a central portion unconfined over bothupper and lower surfaces thereof, and an armature pivoted between saidpole pieces near one of its ends and arranged to engage said resilientblock at the other of its ends at a point of said central portion ofsaid block intermediate said edges.

8. In a sound producer, the combination of a pair of magnetic polepieces, an armature pivoted between said pole pieces at one of its ends,a resilient block, and means to clamp said resilient block alongrelatively narrow portions thereof adjacent two opposite edges thereof,said block having a central portion unconfined over both upper and lowersurfaces thereof, the other end of said armature being in contact withsaid block at a point of said central portion intermediate said edges,the dimension of said central portion of said block between said edgesalong the direction of travel of said armature being large as comparedto the length of said travel.

9. In a sound producer, the combination of a pair of magnetic polepieces each having aledge formed thereon, a resilient member mounted onsaid ledges of said pole pieces, and an armature pivoted between saidpole pieces near one of its ends and arranged to engage said member atthe other of its ends.

10. In a sound producer, the combination of a pair of magnetic polepieces having extensions forming an air gap, an armaturev adapted tovibrate in said air gap, a ledge formed on each of said extensions andspaced from the end of said extension adj acent said armature, aresilient member mounted on said ledges, said armature being pivotedbetween said pole pieces near one of its ends and arranged to engagesaid member at the other of its ends.

11. Ina sound producer, the combination of a pair ofmagnetic pole pieceseach having a ledge formed thereon, a resilient member mounted along twoopposite edgesthere of on said ledges, means including-an adjustingplate connected to said pole pieces to clamp said resilient member tosaid ledges, and an armature pivoted between said pole pieces near oneof its ends and arranged to engage said resilient member at the other ofits ends.

12. In a sound producer, the combination of a casing member, a magnethaving a pair of pole pieces forming an air gap therebetween, each ofsaid pole pieces having an upper plane surface adapted to engage with.

said pole pieces being laterally adjustable through a small distancerelatively to each other across said plane ends of said magnet to varythe length of said air gap, and means to fasten said pole pieces to saidcasing member. a V

13. In a sound producer, the combination of a magnet having two planeend surfaces, a pair of pole pieces each having a plane contact surface,each of said contact surfaces being laterally slidable across adifferent one of said end surfaces to adjust the distance apart of saidpole pieces, and means independent of said magnet surface and said polepiece surfaces to maintain each of said a lower plane end surface ofsaid magnet,

pole piece surfaces in adjusted position in contact with a diiferentoneof said magnet surfaces.

14. In a sound producer, the combination of easing means, a magnethaving a. pair of pole pieces, each of said pole pieces havingan upperplane surface engaging a plane end surface of said magnet, each of saidsaid casing means to assist the magnetic attraction between said magnetand said pole pieces in maintaining said pole piece surfaces in adjustedposition in contact withsaid magnet surfaces.

In witness whereof, I have hereunto set my hand this 20th day of April,1928.

JAMES KENDALL.

